The present invention relates to inorganic laser glass compositions, particularly phosphate laser glass compositions, which exhibit athermal behavior under high thermal loading, good water and chemical durability and high gain.
Silicate laser glass compositions were developed to replace natural or synthetic laser crystals for commercial and military applications. More recently, phosphate laser glass has generally replaced silicate laser glass in many applications because of the high gain potential of phosphate systems and the inherent disadvantages of silicate systems, as disclosed in U.S. Pat. No. 4,075,120 and the copending application of the assignee, Ser. No. 877,606, now U.S. Pat. No. 4,248,732.
Laser rods have been limited in their ability to generate high average brightness by thermally induced optical distortions. These distortions appear as an increase in beam divergence, accompanied by depolarization of the beam. The distortion of the beam appears to vary directly with the input power to the laser and results in both a change in the cavity "Q" and a degradation and eventual failure of the laser to produce a single pulse, where the laser includes a polarization-sensitive "Q"-switch. Temperature variations are necessarily created in a laser rod or the like during lasing; these temperature variations induce distortions of the optical paths within the laser rod, especially when accentuated by periodic pulsing of the laser.
The prior art has attempted to develop "athermal" laser glass compositions by balancing the constituents of the glass to obtain a negative temperature coefficient of refractive index, dn/dt, based upon the following equation: EQU W=dn/dt+.alpha.(n-1)
wherein, W is the thermo-optic constant, .alpha. is the coefficient of thermal expansion and n is the index of refraction. The change in the thermo-optic constant in a laser rod based upon changes in temperature, is then, as follows: EQU .DELTA.W=.DELTA.t[dn/dt+.alpha.(n-1)].times.L
wherein, .DELTA.t is the change in temperature and L is the length of the laser rod or lasing body. Unfortunately, several other factors affect the lasing properties of a laser rod or the like, including Young's modulus, etc.
For example, the optical distortion of a plane wave P, averaged for two polarizations, is given by the following formula: ##EQU1## wherein, E is Young's modulus, .mu. is Poison's ratio, C.sub.1 is the photoelastic constant of the glass, perpendicular to stress and C.sub.2 is the photoelastic constant parallel to the stress.
The birefringence Q may be determined from the following equation: ##EQU2## wherein, C.sub.1 -C.sub.2 is generally accepted as the stress-optic coefficient.
Both experimental and theoretical studies have been performed relating these thermo-optic constants with lasing characteristics. For example, certain tests have established that the distortions of the optical paths inside a lasing element is due to the inhomogeneous temperature field, which is strongly dependent on the average temperature in which the active element is operated. For each type of glass, there is a temperature at which the values of the thermo-optic distortions in one polarization are minimal and are governed only by the temperature drop inside the active elements and the temperature coefficient of the thermo-active constants, see Solid-State Laser Engineering, Koechner (1976) Springer-Verlag. The relation of the thermo-optic constant Q above was reported in Sov. J. Opt. Technol. Mak, et al., Vol. 38, 553 (1971) which concluded that the output energy per pulse decreases when the average pump power is increased and the rate of this decrease is dependent upon the thermo-optic constant Q, which represents the birefringence of the glass.
As described above, each of the thermo-optic constants W, P and Q are composition and temperature dependent and each may be made individually positive, negative or theoretically zero. However, the combination of W, P and Q, acting together, determine the optical distortion of the laser glass at each temperature profile, within the laser rod. There exists for each combination of the thermo-optic constants, a given temperature profile for which the optical distortion is a minimum.
Thus, contrary to the teaching of the prior art, it is not possible to achieve an athermal laser glass merely by adjusting one or more constituents to achieve a negative temperature coefficient of refractive index. The factors are too complex, requiring an experimental approach based upon all of the known thermo-optic constants.
Another problem with many laser glass compositions suggested by the prior art has been poor water durability. In commercial applications, the temperature of the laser rod is reduced or maintained by a cooling fluid, generally water. The laser rod disclosed herein is water cooled. A laser rod must therefore be stable in the cooling medium.